Add support for recaptcha during the create account flow

If the feature is enabled and recaptcha keys are given in config, then a recaptcha box is displayed in the UI when creating a user and a recaptcha response code *must* be sent with the create API call for it to succeed.

templates/base.html

@@ -74,6 +74,10 @@
       </script>
     {% endif %}
 
+    {% if recaptcha_key %}
+    <script src="//www.google.com/recaptcha/api.js?render=explicit&onload=vcRecaptchaApiLoaded" async defer></script>
+    {% endif %}
+
     {% if munchkin_key %}
       <script type="text/javascript">
       (function() {